The subject matter of this invention relates generally to VAR generators and in particular to static VAR generators.
In recent years there has been a greatly increased demand for power factor correction and control in utility and industrial systems due to the use of electrical machines, the rapidly increasing popularity of the electric arc furnace in steel production, and the general acceptance of naturally commutated thyristor drives and power controllers. One particular kind of power factor correction means is the static VAR generator system employing a fixed capacitor and thyristor controlled inductor in each phase of a single or polyphase AC system. This is often utilized in conjunction with an electric arc furnace system. An electric arc furnace often represents a rapidly and in a random manner varying generally unbalanced load with poor lagging power factor. The large reactive current swings cause corresponding voltage drops across the reactive impedance of the system resulting in a variation in the terminal voltage at the critical bus which is common to both the arc furnace load and other utility loads. The utilities' major concern with regard to this voltage variation is the resulting variation in the light output of electric lamps connected to its buses, generally referred to as flicker, and the sensitivity of the human eye to the flickering lamps. To reduce the flicker the utilities usually require that the arc furnace load be complemented with appropriately controlled reactive power generators of VAR generators so that the net reactive power variation of the AC system is kept below a specified level. Other types of loads which require compensation are radar transmitters and receivers which use the transmitting tube known as the klystron tube. In U.S. Pat. No. 3,424,970 entitled "Self-Regulated Rapid Response Phase Control Rectifier" issued to James A. Ross on Jan. 28, 1969 a method of controlling electrical rectifiers by alternating the period of current flow therethrough and the means for carrying out method is taught. In the method and apparatus previously described the phase to neutral voltages at the input of a three phase, bridge controlled rectifier are compared to a fixed reference voltage and the appropriate controlled rectifiers are fired when the instantaneous values of these phase to neutral voltages become successively less than the reference voltages. The average output voltages are linear functions of the reference voltage over all conduction angles with which the input filter reactor is large enough to maintain continuous conduction. In U.S. Pat. No. 3,551,799 entitled "Mains Voltage Stabilizing Apparatus Providing Constant Reactive Current" issued to F. Koppelmann on Dec. 29, 1970 an apparatus for stabilizing the voltage in a power supply main having a load which draws varying amounts of electric current is taught. The apparatus includes an electrically variable inductance connected to the main line either in series or in parallel with the load and an electronic control circuit which regulates the instantaneous value of inductance of the variable inductance. The instantaneous value of inductance is chosen so that the reactive current drawn by the variable inductance will supplement the reactive current drawn by the load in such a way as to maintain the total reactive current drawn from the main line of an approximately constant value. In U.S. Pat. No. 3,693,069 entitled "Gating Control For Static Switching Arrangement With Improved Conduction Angle Balancing Means" issued to F. W. Kelly, Jr. et al on Sept. 19, 1972, a gating control is taught for a switching arrangement comprising a pair of thyristors which are alternately gated into conduction by gating pulses for predetermined conduction angles. U.S. Pat. No. 3,769,440 entitled "Current Monitoring Systems For Arc Furnace Conductors" issued to D. J. Goodman on OCT. 30, 1973, teaches a system for monitoring the voltage on the conduction cables of an arc furnace. If any of the cables for any reason has less than its proportional amount of current this condition is indicated by a reading on a voltmeter so that an operator can take corrective action. It would be advantageous if a high speed control system could be divised for utilization within a VAR generating circuit for a variable impedance load, such as an arc furnace, to thereby control the compensating current supplied to the AC system i.e. variable impedance load to make it appear at the input terminals of the system that the system is operating under constant reactive load and phase conditions. It would be advantageous if this system were highly mathematically oriented so that phase load currents could be utilized and converted mathematically by integration to firing angles for thyristors. It would be advantageous if the firing angles could be varied according to the magnitudes of the parameters being sensed. It would also be advantageous if the amount of reactive current generated did not have to be constant but could vary according to the conditions of the load. It would be advantageous if the control system acted with high speed.